1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a transflective LCD device.
2. Description of Related Art
In general, liquid crystal displays are divided into transmissive LCD devices and reflective LCD devices according to whether the display uses an internal or an outer light source.
A typical transmissive LCD device comprises a liquid crystal panel and a back light device. The liquid crystal panel includes upper and lower substrates with a liquid crystal layer interposed therebetween. The upper substrate has a color filter, and the lower substrate has a thin film transistor (TFT) as a switching element. An upper polarizer is arranged on the upper substrate of the liquid crystal panel, and a lower polarizer is arranged between the lower substrate of the liquid crystal panel and the backlight device.
At this time, the two polarizers have a transmittance of 45%, the two substrates have a transmittance of 94%, the TFT array and the pixel have a transmittance of 65%, and the color filter has a transmittance of 27%, respectively. Therefore, the transmissive LCD device gets to have a transmittance of about 7.4% as seen in FIG. 1, which shows transmittance after light passes through each of the layers. For such a reason, the transmissive LCD device requires a high brightness, and thus electric power consumption by the backlight device increases. In order to supply sufficient power to the backlight device, a relatively heavy battery is employed, and there still exists a problem that the battery can not be used for a long time.
In order to overcome the problem described above, the reflective LCD has been developed. Since the reflective LCD device uses ambient light, it is easy to carry. Also, the reflective LCD device is superior in aperture ratio than the transmissive LCD device.
FIG. 2 is a plan view illustrating a typical reflective LCD device. As shown in FIG. 2, the reflective LCD device 100 includes gate lines 6 and 8 arranged in a transverse direction, data lines 2 and 4 arranged in a longitudinal direction perpendicular to the gate lines 6 and 8, and thin film transistors “S” (TFTs) near cross points of the gate line 8 and the data line 2. Each of the TFTs “S” has a gate electrode 18, a source electrode 12 and a drain electrode 14. The data electrode 18 extends from the data line 2, and the gate electrode 18 extends from the gate line 8. The reflective LCD device 100 further includes reflective electrodes 10. The reflective electrode 10 is electrically connected with the drain electrode 14 through a contact hole 16 and is made of a metal having a good reflectance.
By the way, the reflective LCD device has a problem that it is affected by its surroundings. For example, the brightness of ambient light in an office differs largely from that of the outdoors. Also, even in the same location, the brightness of ambient light depends on the time of day (e.g., noon or dusk).
In order to overcome the problem described above, a transflective LCD device has been developed. FIG. 3 shows a conventional transflective LCD device. As shown in FIG. 3, the conventional transflective LCD device includes lower and upper substrates 50 and 60 with a liquid crystal layer 80 interposed therebetween. The upper substrate 60 has a color filter 61, and the lower substrate 50 has a switching element (not shown), a pixel electrode 54 and a reflective electrode 52. The reflective electrode 52 is made of a opaque conductive material having a good reflectance and includes light transmitting holes 53 formed therein. The transflective LCD device further includes a backlight device 70. The light transmitting holes 53 serve to transmit light 112 from the backlight device 16.
The transflective LCD device is operable in both a transmissive mode and a reflective mode. First, in the reflective mode, the incident light 74 from the upper substrate 22 is reflected on the reflective electrode 52 and directs toward the upper substrate 22 again. At this time, when the electrical signals are applied to the reflective electrode 52 by the switching element (not shown), phase of the liquid crystal layer 80 varies and thus the reflected light is colored by the color filter 61 and displayed in the form of colored light.
Further, in the transmissive mode, light 72 generated from the backlight device 70 passes through portions of the pixel electrode 54 corresponding to the transmitting holes 53. At this time, when the electrical signals are applied to the pixel electrode 54 by the switching element (not shown), phase of the liquid crystal layer 80 varies. Thus, the light 72 passing through the liquid crystal layer 80 is colored by the color filter 61 and displayed in the form of images with other colored lights.
As described above, since the transflective LCD device has both the transmissive mode and the reflective mode, the transflective LCD device can be used without depending on the time of day (e.g., noon or dusk) and has advantages that it can be used for a long time with consuming a low power.
However, since the reflective electrode 52 has a plurality of the transmitting holes 53, the conventional transflective LCD device has a very low light utilizing efficiency compared to the reflective LCD device or the transmissive LCD device. For example, it should be noted that the number of the circular-shaped light transmitting hole 53 having a diameter of ΔL is “n” an area of one light transmitting hole 53 is “(πΔL2)/4”, and thus a total area of all light transmitting holes 53 gets to be “n×(πΔL2)/4”. As a result, the light utilizing efficiency of the transmissive mode and that of the reflective mode are in inverse proportion according to the total area of the light transmitting holes 53. In other words, if a total area of the light transmitting holes 53 is reduced in order to increase the light utilizing efficiency in the reflective mode, in proportion to the reduced area of the holes 53, the light utilizing efficiency in the transmissinve mode gets to be reduced.
For the foregoing reasons, there is a need for a transflective LCD device having a good utilizing efficiency in both the transmissive mode and the reflective mode.